How we onboard/allow apps to use IAM Identity Center

How to onboard users and Groups from the external IdP to AWS Identity Center, as a Bulk.

I came across this in IIC documentation "Users who have not yet been assigned to an application or AWS account cannot be provisioned into IAM Identity Center. To synchronize users and groups, make sure that they are assigned to the application or other setup that represents your IdP’s connection to IAM Identity Center". This means we cannot sync users independent of the app assignments.

that statement is confusing...how does AWS even check that....I could manually create any random user in Identity Center. But in the following Docs link says https://docs.aws.amazon.com/singlesignon/latest/userguide/provision-automatically.html

**For SCIM synchronization to work, every user must have a First name, Last name, Username and Display name value specified. If any of these values are missing from a user, that user will not be provisioned. **